Heat exchangers often have a distributor tube whose external surface is provided with cooling fins. The distributor tube is typically a steel tube coated with a metal having good heat conduction, such as aluminum. The cooling fins themselves also generally comprise aluminum because of its good heat conductivity and low weight.
The bond between the distributor tube and the cooling fins is generally created through a brazing process. Traditionally, the distributor tube was cladded with aluminum or an aluminum-silicon alloy. However, when producing an aluminum cladded tube, intermetallic layers form between the cladding and the base steel material. After forming the cladded tube, the tube must be cleaned for brazing. This cleaning typically involves water based cleaners which need to be dried before the brazing process. The cleaned and cladded tube is then subjected to a secondary heating such that the tube is bonded to the aluminum fins at a brazing temperature of approximately 600° C. Secondary heating increases the thickness of the brittle intermetallic layer which is subject to cracking. Also, during this secondary heating process, the aluminum layer can be thinned from capillary action during brazing to the point where it does not provide sufficient long term corrosion protection for the base steel material.
Accordingly, there is a need for brazing compositions which: (1) eliminate the need for aluminum cladding prior to the brazing process; (2) eliminate the need for a drying step in the brazing process: (3) eliminate the need for secondary heating in the brazing process; (4) reduce or eliminate the intermetallic layer between the cooling tin and the distributor tube; and (5) provide long term corrosion protection.
Embodiments of the present invention are designed to meet these ends.